At the beginning of 1939 casualty administration within the Air Ministry formed one of the duties of S7(d), a branch of Secretarial Division S7. Although part of the Department of the Permanent Under-Secretary of State for Air, S7 was allotted to the Department of the Air Member for Personnel (AMP) and responsibility for the functioning of this division and its constituent branches was therefore to an extent divided between the Permanent Under-Secretary (via the Principal Assistant Secretary (Personnel) and the First Deputy Under-Secretary of State) and the AMP.
Following the outbreak of war in September 1939, it was decided that a dedicated casualty branch should be formed, similar to that which had existed within the Air Ministry at the end of the First World War. On 16 October 1939 an Air Ministry Office Memorandum was circulated noting that approval had been granted for the transfer of those elements within S7(d) 'responsible for duties in connection with casualties' to a new branch designated P4(Cas). This new branch fell within the Directorate of Personal Services and as such was responsible solely to the AMP, via the Director of Personal Services. P4(Cas) was located initially in Adastral House, Kingsway and moved to 73-77 Oxford Street in late 1942.
The staff of P4(Cas) were called upon from the outset to undertake research into the whereabouts and fate of RAF personnel listed as missing. As the day-to-day work of this branch mounted, it was decided in 1941 that a 'Missing Research Section' should be created within P4(Cas) charged primarily with conducting these detailed and time-consuming investigations. Although all of the research undertaken prior to 1944 had to be conducted from London, the Allied liberation of Europe enabled search teams to operate in the field for the first time. The first of these, No 1 Missing Research Section was formed in Paris in December 1944. By March of the following year a 'RAF and Dominion Air Forces' Missing Research and Enquiry Service' (MRES) had been established 'for the purpose of research and enquiry, in liberated territories and those occupied by Allied forces, into the circumstances of air crews reported missing of whom no previous trace has been found. The service will also endeavour to obtain additional information to supplement that already received.'
During 1945 a total of eight Missing Research Sections were established across Western Europe. However, such was the scale of the task confronting the MRES that in July 1945 it was decided that this organisation should be greatly expanded. By April 1946 the eight Missing Research Sections had been replaced by four larger Missing Research and Enquiry Units (MREUs) covering Western and Central Europe. Subsequently, a fifth MREU was created in Italy to conduct investigations in Southern Europe and the Mediterranean. Missing research activities were also conducted in the Far East.
Although the MREUs were attached for administrative purposes to RAF formations in the country or region in which they were to operate, the MRES remained under the direction of the Director of Personal Services and its work was coordinated by P4(Cas).
Following the disbandment of the MRES on 30 September 1949 the casualty branch continued to be responsible for all aspects of research into those still listed as missing and a small number of 'Missing Research and Enquiry Liaison Staffs' (MRELS) posts remained after this date.
On 19 August 1946 control of the casualty branch was transferred from the Directorate of Personal Services to S7 and P4(Cas) was redesignated S7(Cas). Although this would appear to have marked a reversion to the pre-war pattern of casualty administration, the Director of Personal Services continued to maintain an 'interest in the work of the Casualty Branch and of the Missing Research and Enquiry Sections abroad' and responsibility for this oversight function was vested in the Deputy Director of Personal Services (2). In March 1947 S7(Cas) relocated from 73/77 Oxford Street to 2 Seville Street, Knightsbride, SW1, and in the following month the branch switched from S7 to Secretarial Division S14, being redesignated S14(Cas) with effect from 28 April 1947. As part of a programme to transfer Air Ministry branches from Central London to the 'periphery' the personnel of S14(Cas) moved from Seville Street to the government offices at Stanmore in April 1949. Secretarial Divisions S7 and S14 were combined under the title S7 with effect from 1 February 1956 and as part of this reorganisation the responsibility for casualty administration was transferred to the RAF Officers Appointments and Record Office (OAR), the casualty branch now becoming AR9(Cas).